It's not open source. The open source definition is about more than being able to see source code. The usual term for the latter is "source-available" which tells you nothing about what you're actually able to do with it.

Another term I use is "proprietary source available software" (PSAS) as a mirror to "free, open source software" (FOSS). Note that the "free" in that term refers to free software (libre) not free-of-cost (gratis). The free software definition and open source definition generally overlap so most licenses that are one will also be the other. In other words, "open source but not FOSS" is not a thing (and is generally a misnomer for PSAS).

As an analogy, you wouldn't say a store is "open" just because you can look in the windows and see things.